A realization method of wide-stopband substrate-integrated waveguide (SIW) diplexers and dual-band bandpass filters (BPFs) with widely realizable frequency ratios is presented using modified intrinsic modes suppression technique. Design procedures of dual-band and wide-stopband characteristics for different frequency ratios are systematically summarized by sufficiently using the limited design degrees of freedom. The mixed dual-and single-mode cavities and the offset external coupling slots could help to flexibly control the dual-band frequencies and bandwidths. Meanwhile, the internal coupling windows could produce predictable wide-stopband performance when properly arranged at the positions of weakest electric fields of specific unwanted spurious modes determined by the frequency distributions for the required frequency ratio. Two SIW diplexers and an SIW dual-band BPF with frequency ratios of 1.225, 1.5, and 1.425 are synthesized and designed for demonstration, extending the stopband up to 1.79f₀ with better than 24 dB rejection level, 2.05f₀ and 2.34f₀ with better than 20 dB rejection level, respectively.